The present invention relates to a process for the introduction of a condensed liquid into a reactor for the gas-phase (co)polymerisation of ethylene and/or propylene in a fluidised bed.
It is known to polymerise one or more monomers in the gas phase at a pressure greater than atmospheric pressure in a fluidised-bed reactor in which polymer particles being formed are maintained in the fluidised state by virtue of a reaction gas mixture which contains the monomer or monomers to be polymerised and flows as an ascending stream. The polymer thus manufactured in the form of powder is generally withdrawn from the reactor so as to maintain the bed with a more or less constant volume. A preferred industrial-scale process uses a fluidising grid which distributes the reaction gas mixture through the bed and which serves as a support for the bed should the flow of ascending gas be cut off. The reaction gas mixture leaving the top of the fluidised-bed reactor is recycled into the base of the latter beneath the fluidising grid via an external circulation pipe provided with a compressor.
The polymerisation of monomers is an exothermic reaction. It is therefore necessary to provide a means suitable for cooling the bed so as to extract the heat of polymerisation therefrom. The preferred method for the fluidised-bed polymerisation of ethylene and/or propylene consists in cooling the reaction gas mixture below the polymerisation temperature, thereby making it possible, while this fluidising gas is passing through the bed, to compensate for the excess heat generated by the polymerisation. Thus, during its return, the reaction gas mixture is generally cooled using at least one heat exchanger placed in the external circulation pipe so as to extract the heat produced by the polymerisation reaction and to maintain the polymerisation temperature at the desired level.
Attempts have been made, most particularly in recent years, to optimise the gas-phase polymerisation process so as to increase the production of polymer in existing plants. The rate of polymer production has therefore been studied, namely in terms of efficiency by weight of polymer produced per unit volume of the reactor and per unit time (kg/h/m3). In commercial fluidised-bed reactors of the above mentioned type, it is known that the rate of production depends directly on the rate of removal of heat generated in the reactor. This rate of removal may be increased, for example by increasing the velocity of the fluidising gas and/or by reducing the temperature of the fluidising gas and/or by increasing the heat capacity of the fluidising gas.
The Patent Application WO 94/28032 discloses a gas-phase olefin polymerisation process in which the recycling gas stream is cooled to a temperature low enough to form a liquid and a gas. By separating the liquid from the gas and by introducing the liquid directly into the fluidised bed, the total amount of liquid introduced into the fluidised-bed reactor may be increased, which in turn allows better cooling of the bed by evaporation and therefore allows higher productivity levels to be achieved. Many methods for introducing the liquid into the fluidised bed have already been described.
The Patent Application WO 98/18548 discloses nozzles to inject a liquid in a fluidised bed comprising a pressurised liquid inlet and a liquid outlet in which a mechanical device is provided within the liquid outlet to atomise the liquid and the liquid outlet is provided with a spray-forming zone. When no liquid is injected, a gas is often used and injected instead of the liquid to avoid blockage by the powder in the fluidised bed. The introduction of a gas is often a constraint as it generates a larger purge out of the fluidised bed reactors in which those nozzle are used.
The U.S. Pat. No. 2,164,411 discloses a nozzle for the injection of a fluid under pressure, such as air or steam, into a container for pulverulent, granular or like material to aerate such material in and discharge the material from the container. The nozzle comprises a casing having an inlet opening adapted to be connected to a source of fluid under pressure, an outlet and a valve to control the discharge of the fluid through the casing. The casing of the nozzle comprises a stem slidably mounted in the casing, having a valve head fixed at one end to be seated against the valve set at the end of the casing. At the other end of the stem is fixed a piston head mounted within the casing, allowing the passage of the fluid by moving the valve head away from the valve seat. A mean to urge the valve head to be seated against the valve set is provided by a spring in order to shut off the inlet of the casing from the source of fluid under pressure. The nozzles described in the U.S. Pat. No. 2,164,411 have never been used for injecting a liquid, and even less for introducing a liquid into a fluidised bed.
The Patent Application WO 96/20780 discloses a method to inject a liquid directly into a fluidised bed by the use of a nozzle using an atomisation gas, for which the horizontal penetration of the liquid into the fluidised bed is in the range of 250 to 25000 and the pressure drop across the mixing chamber is in the range of 0.88 to 1.5 bar. It is highlighted that the relation between the area of the outlets and the flow rate through the nozzle as well as maintaining the required pressure drop is important in achieving the optimum penetration and dispersion of the liquid. The nozzles referred in the Patent Application WO 96/20780 do not allow a control of the liquid penetration. Only the cross section area of the orifices can be adapted to achieve the optimum penetration and dispersion of the liquid in accordance to the range of flow rate targeted.
The nozzles used to introduce a liquid in into a fluidised bed disclosed in the prior art have, for most of them, a problem linked to the length of the liquid jets that are generate and their impact with the fluidised bed. In order to maintain a bed of powder in a fluidised state, it is important that anywhere within the fluidised bed, the concentration of liquid does not go beyond a limit, above which the powder agglomerates and cannot be maintained fluidised. The present invention seeks to limit the length of the liquid jets in the fluidised bed to avoid impingement of the jets with, and accumulation of the liquid at, the internal wall of the fluidised bed reactor. The present invention also seeks to ensure that the length of the liquid jets are long enough to allow a good dispersion of the liquid in the fluidised bed and to avoid accumulation of the liquid near the nozzle outlet orifices.
A method has now been found to introduce a liquid into a fluidised bed reactor with at least a nozzle, this method allowing to limit the length of the liquid jet(s) in the fluidised bed, and more generally to control the length of the liquid jet(s) in accordance with the liquid flow rate through the nozzle. This method is particularly adapted for the introduction of a liquid in a continuous gas-phase polymerisation process in gaseous phase.
The invention relates to a process for the introduction of a liquid into a continuous process for the gas-phase polymerisation of an olefin monomer chosen from (a) ethylene, (b) propylene, (c) a mixture of them, and one or more other alpha-olefins in combination with (a), (b) or (c), in a fluidised-bed reactor, by continuously recycling the gas phase which supports the fluidised bed and passes through the latter, the said gas phase having been heated in contact with the polymerisation catalyst under reactive conditions and with polymer particles being formed, by cooling the said recycled gas phase, process characterised in that the liquid is introduced by means of at least one nozzle comprising a feed pipe (1) and a sleeve (2) sliding inside and at the end of the said pipe, the position of the said sleeve (2) along the feed pipe (1) being determined by the pressure of the feed liquid (3) and the return force of a preloaded spring (4), the said spring having one end fixed to the sleeve (2) and the other end to the feed pipe (1), the sleeve (2) being provided with one or more recesses (10) along its surface for sliding with the feed pipe (1), the said recesses (10) emerging in a groove (11) around the circumference of the sleeve (2) and below a circular stop (8) integral with the same sleeve (2), and characterised in that,
i) when the pressure of the feed liquid (3) is not high enough to move the sleeve (2), the said stop (8) rests on a rim (9) of the end of the feed pipe (1) making it possible for the introduction of the said liquid (3) to be stopped or for it to be kept stopped, the said rim (9) and the circular stop (8) of the sleeve each being provided with a bearing surface (12) and (13), the contact area of which being minimised in order to allow good sealing, and,
ii) when the pressure of the feed liquid (3) is high enough to move the sleeve (2), the said liquid (3) is conveyed by the recesses (10) as far as the groove (11) in the sleeve in order to be introduced towards the outside between the bearing surfaces, on the one hand, of the rim (9) of the end of the feed pipe (1) and, on the other hand, of the stop (8) of the sleeve (2).